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Month: September 2015


Clipboard01nnnnI often want to listen my favorite music at nighttime without disturbing the neighbors….In the modern apartment with thin walls it’s impossible…….Yes I think I need a good headphone and a suitable amplifier.
But common low cost op-amps found in most consumer audio equipment just don’t have the grunt force and the finesse to actually let the headphones do what they do, which is get out of the way and deliver great music. Well, this was a first for me, building an headphone amp, so I decided I’d prototype an amp and see how it goes. The goal was as cheap as possible, with the minimum of hassle with the focus on the circuit and sound, not a final product so to speak. Here’s how it went. Vacuum tube circuit, have a simplicity unbeatable from any transistor amplifier, however, there are not many projects that use a single active component ( minimalism philosophy ).
This is a single ended transformer coupled amplifier that employs a single pentode tube D3A as active device. There are other excellent candidates : E55L, EC8020, 3A167M, 437A but all are more expensive. Normally a pentode in triode strapped reveal unlinear curve. Not so with D3A. This tube in triode mode give excellent data: amplification factor=70, trasconductance=46mA/V, plate resistance=1.5k; plate dissipation 4,5W and very linear anode curve. The perfect choice for my cheap transformer load, compact, easy-to-building, SE headphone amp.
Take a look at schema: I chose the fixed bias for D3A. Put a DC millivoltmeter across a 1 ohm resistor ( TP ) and turn VR bias trimmer until you read 16 mV ( 16 mA in the tube ) with 170 V in the anode. I noted that this is the operating point for the “right sound” with astounding results in terms of transparency and musicality ( low THD at measurement ). If you want, you can easily use the VR pot for little change and find your optimal “sweet spot” . In this application I chose Lundahl LL1689/18mA as output transformer in ALT/R ( 18:1 ) configuration ( ). A primary inductance of 90 H ensure good low frequency response. Use alternative configuration ( 18:4, 18:2 ) for best match your headphone impedance. Put a GS ( grid stopper ) carbon composite 470-1K ohms resistor on tube grid pin to prevent nasty hf-influences. In power supply I suggest to use tube rectifier like 5Y3GT followed by LCLC type regulator. For the lowest hum and best L/R separation use a Mosfet capacitance multiplier stage. The 0,5 uF capacitors in the schema must be the highest possible quality ( polystyrene or polypropylene type ). About the power transformer the secondary windings must have: 240+240 V at 100mA for HT, 6,3 V at 1 A for 6C45Pi filament, 5 V at 2 A for 5Y3GT and 5 V at 100mA for negative bias.
Sound quality
This really was a big surprise to me! The amplifier has a most detailed, warm and very good balanced sound. Voices are very fine lifelike and excellent dynamics attacks. You can listen to this amp for days and days without your ears getting tired – music comes through fast and accurate without any buttery soupiness, but still open and airy without a hard edge. So for little money one can listen to expensive tube sound!
As mentioned above there are other excellent tube candidates : E55L, EC8020, 3A167M, 437A and so on.. but all are more expensive. I found a small triode with excellent data: amplification factor=52, trasconductance=45mA/V, plate resistance=1k; plate dissipation 7,8W the 6C45Pi. In this case the audio output transformer must have a 5Kohms primary impedance and a secondary that well match the impedance of your headphone. A good choice can be Sowter 9351 single ended output transformer ( ): it has a 5K ohms primary and 4 separate secondary windings configurable for 40-150-300-600 ohms. Adjust the bias trimmer for 35 mA in the tube with 170 V in the anode. I noted that this is the best operating point for 6C45Pi. If you want, you can easily use the VR pot for little change and find your optimal “sweet spot” . 6C45Pi has 2 grid pin connection ( 2, 8 ). Use a GS ( grid stopper ) carbon composite 470-1K ohms resistor on each pin to prevent nasty hf-influences. The power supply may be the same of D3A version or adopt a more cheaper type as tube rectifier 5Y3GT followed by CLC type configuration regulator. First capacitor MUST BE paper in oil type 4 uF/ 400-600V.

K2 Klipschorn (super) klone…

This is my reference speakers……
I heard hundreds of speaker systems but at last….I always return at my Klipschorn super clone. Why super ? OK bass bin is almost identical to original Klipschorn, also fitted with original K33E Klipsch woofer but mid-high section is very different.



I use high-end expensive component like Gauss HF4000 as midrange assembled with JBL 2350 aluminum horn. Gauss HF4000 2”compression drivers are very impressive components: heavy (13 Kg ), alnico magnet, large aluminum diaphragm, wide frequency response ( 500 to 15.000 Hz ) excellent matched with JBL 2350 40×90 radial horn with 220 Hz taper cutoff frequency. This excellent heavy ( 16.3 Kg ) aluminum horn measure 80,3 x 50,8 x 20,3 cm and so match very well with size of bass cab Klipschorn. For high range I chosen the Coral H-104 horn tweeters.
This big ( 3.5 Kg ) alnico tweeter is mounted in the same vertical plane of the Gauss driver diaphragms for best time alignment mid-high response. The crossover filter is an variant of Klipsch type-A. I choose first order slope crossover for his best sound. I tried various crossover type also electronic ( tube and solid state ) but I returned back to the simplest 6dB/oct in the schema. Be carefuI on quality components: for mid-range I use paper in oil capacitors, in the high section copper MKP I found has more detailed sound. 10 Ohms resistor must be metal/oxide 1%, 10W. The 3,6mH inductor for low pass filter must be high quality, low RDC, ferrite ( or best transformer ) core with almost 1,4 mm copper section.
Mid range level , due very efficient Gauss driver, must be attenuate with an autotransformer ( like model 3636 from B&K Sound ). In my system I use pin 0-5 as input and 0-3 as output. These speakers have a first rate SPL factor. 3 W of 2A3 SE tube amplifier, are sufficient to produce a powerfull, open, deep sounstage. It is very difficult for any speaker to play music as a live session….. these speakers are among the very few that succeed in this uneasy task.

Die zauberflote‭!!‬…

Zauberfline2uuuuuuuuDie zauberflote‭!!‬…it‭’‬s‭ ‬a magic flute now I thought when I listened for first time this DAC.
‭ ‬Yes,‭ ‬the original Musiland monitor‭ ‬02Us was a honest good cheap USB DAC but nothing more. ‭ ‬All low cost DAC or CD player use a modest analog op amp based output stage.‭ ‬But a quick look in the schematic and specs DAC I saw a very good digital part‭ ‬:‭ ‬high speed USB interface,‭ ‬SNR and dynamic range of‭ ‬113dB,‭ ‬good PCM1793‭ ‬DAC,‭ ‬excellent‭ ‬Windows driver and so on.‭ ‬Well,‭ ‬this is a good,‭ ‬true‭ ‬24‭ ‬bit/192kHz digital device.‭ ‬So it‭’‬s worthwhile a serious heavy upgrade:‭ ‬a tube push-pull output stage.‭  ‬In this role a good candidate is the Russian tube‭ ‬6H30:‭ ‬very linear,‭ ‬low plate resistance,‭ ‬adequate gain‭…‬a good choice for transformer load stage. ‭ ‬The tube new circuit must replace the op-amp output stage on the Musiland board taking the input signals as in the schema below.‭ ‬CCS‭ ( ‬constant current source‭) ‬on the cathodes‭ ‬6H30‭ ‬is based on DN2540‭ ‬N channel depletion mode vertical mosfet.‭ ‬With the source resistor in the schema the current is set to‭ ‬40mA‭ ( ‬20mA per tube‭)‬.  
‭ ‬GS is a carbon composite gate stopper resistor:‭ ‬any value from‭ ‬220‭ ‬and‭ ‬1k ohms 0,5W is OK, for the other resistors use 1% precision metal or carbon type 0,5W.‭ ‬50‭ ‬ohms multiturn trimmer allows to balance current in primay push-pull transformer‭ ( ‬e.g.‭ ‬set the equal‭ ‬6h30‭ ‬anode voltage‭)‬.‭ A poor DC current balance might saturate the core and reduce the inductance of the windings ( worse low frequency response ). ‬Power supply‭ ‬it‭’‬s a very fine tube rectifier,‭ ‬choke input circuit ( choke-fed supply are much quieter due to the choke slowing down the rate-of-charge of the main 47uF cap ) and‭ ‬need a transformer with‭ ‬3‭ ‬windings:‭ ‬260+260V at‭ ‬100mA for HT,‭ ‬5‭ ‬V at‭ ‬2A‭ (‬min‭) ‬for‭ ‬5Y3GT filament,‭ ‬6,3‭ ‬V at‭ ‬2A (min‭) ‬for‭ ‬6H30‭ ‬filament.   ‭ ‬Note in Power supply schema,‭ ‬after first‭ ‬47uF cap,‭ ‬the anode voltage are splitted for best R-L separation performance.‭ ‬All capacitors used are MKP type‭ ‬bypassed with‭ ‬0,1uF polystyrene or silver mica.‭ ‬Don‭’‬t forget to ground one leg of‭ ‬6H30‭ ‬filament to reduce hum.